1. Field of the Invention
The present invention relates to charge transfer devices, more particularly to information signal transfer methods operable at a high signal transfer efficiency in the transfer of an information signal in terms of a plurality of signal charges. The invention relates further to charge transfer devices having a means for injecting a plurality of signal charges into semiconductor surface regions.
2. Description of the Prior Art
As is well-known, the charge-coupled semiconductor device is structurally simple and easily manufacturable, and is used in many ways, such as for shift registers, image devices and display devices. This type of semiconductor device has the structure of MIS (Metal Insulator Semiconductor) and typically comprises a semiconductor substrate, an insulating layer disposed on one surface of the substrate, a means for introducing electric charge carriers into the semiconductor substrate, electrodes separately disposed on the insulating layer for storing the electric charge carriers introduced into the semiconductor substrate and for transferring the electric charge carriers along the surface of the semiconductor substrate adjacent to the insulating layer, means connected to the electrodes for applying an electric field for transferring the carriers to the semiconductor substrate, and a means for detecting the carriers transferred.
In the semiconductor substrate, the minority carrier is used as the electric charge carrier since the semiconductor is adequate to generate minority carriers. Hence, when the semiconductor substrate is of N-type, holes are used as electric charge carriers; while, when it is P-type, electrons are used.
The charge-coupled semiconductor device operates in the following manner.
A DC voltage is applied to a first one of the electrodes on the insulating layer so that a depletion region is formed in a surface region of the semiconductor substrate adjacent to the insulating layer corresponding to the first electrode. Since this depletion region appears only in the surface region of the semiconductor substrate immediately below the first electrode to which the DC voltage has been applied, a potential well is formed in such surface region.
In this state minority carriers are introduced into the semiconductor substrate when, for example, a forward voltage is applied to a P-N junction in the device, or an avalanche phenomenon is induced into the metal oxide semiconductor structure, or radiant rays or light beams are applied thereto. These minority carriers are then collected in the potential well.
A DC voltage larger than was applied to the first electrode is applied to a second electrode next to the first one, with the result that a second potential well deeper than the first one is formed under the second electrode.
Because the carriers in the shallow potential well shift to the deep potential well, the minority carriers are transferred to the surface portion in the semiconductor substrate immediately below the second electrode. After the minority carriers have been transferred, the DC voltage applied to the first electrode is cut off and the voltage on the second electrode is lowered to the level of the voltage applied to the first electrode. This shows that the minority carriers which have first been collected in the place immediately below the first electrode are transferred to the portion in the semiconductor substrate immediately below the second electrode.
By repeatedly applying DC voltages in the above manner, it becomes possible to transfer the minority carrier from one electrode to another.
Principles and fundamental structure of the charge coupled semiconductor device are described in detail in "Charge Coupled Semiconductor Devices" by W. S. Boyle and G. E. Smith, The Bell System Technical Journal, Vol. 49, No. 4 (April, 1970), pp. 587 - 593.
The minority carrier is transferred by applying a three-phase pulsed DC voltage to a trio of electrodes as described in the above paper; they may also be transferred by applying a two-phase pulsed DC voltage to a pair of electrodes and changing the thickness of the insulating layer under each of the electrodes.
This charge-coupled semiconductor device has drawbacks. For example, not 100% of the minority carriers can be transferred from the portion of the semiconductor substrate immediately below an electrode to the portion under the adjacent electrode. This leads to an increased loss of the minority carrier, or a possibility of causing the information to be detected inaccurately. Furthermore, increasing the number of electrodes used results in a lower transferring efficiency, and there are limitations on the number of electrodes which can be used for transferring a sufficient amount of minority carriers to be detected as an information signal.